206 ARTHUR DENDY. 



Dermal oxea, like those of Grantia, may also be present 

 (fig. 21). The chamber layer is generally further supported 

 by other quadriradiate spicules, which may have a more or less 

 definite subgastral position (fig. 31), or be irregularly scattered 

 through the chamber layer (fig. 22), or by irregularly scattered 

 triradiates, as in Leucilla prolifera (4). 



The skeleton of Leucilla (Leucandra) cucumis, Haeckel 

 [5), exhibits a further complication of the Leucilla type. The 

 skeleton of the dermal cortex is made up of an outer layer of 

 triradiate spicules, as in Grantia, and an inner layer of quadri- 

 radiates with inwardly directed apical rays. These quadri- 

 radiates help to form a framework for a remarkably regular 

 series of subdermal cavities, and are assisted in so doing by a 

 second, deeper layer of quadriradiates with outwardly directed 

 apical rays. Within this deeper layer of quadriradiates comes 

 the chamber layer of the sponge wall, supported by irregu- 

 larly scattered quadriradiates ; and within this comes the gas- 

 tral cortex, composed of triradiate spicules. In addition to 

 the spicules mentioned, large oxeote spicules are found arranged 

 longitudinally between the dermal triradiates. Illustrations 

 of the skeleton of this remarkable sponge will be found in 

 Haeckel's great work. 



Summary. 



We find, from our survey of the various genera of Hetero- 

 coela, that the starting-point for the development of the 

 skeleton throughout the group (leaving out of account Leu- 

 cascus, which may have originated independently from the 

 Homocoela) is the radially symmetrical skeleton of Lycetta. 

 This primitive radial symmetry is highly characteristic of the 

 group, and is obviously dependent upon the primitive radial 

 symmetry of the canal system. The first great change in the 

 structure of the skeleton is brought about by the development 

 of a dermal cortex, in which a special skeleton is formed. 

 The skeleton of the chamber layer of the sponge wall now 

 begins to vary. This variation is in some cases obviously de- 

 pendent upon the gradual change of the canal system from the 



